Add support for mocking network state and speed

[kenzieschmoll]

This is something we could let users control from DevTools, but I suspect the implementation of this would have to live in dart:io. @bkonyi do you have any ideas on how we could let users throttle their network traffic?

[kenzieschmoll]

related: #969

[srawlins]

+1; @bkonyi it seems like this implementation would primarily live in dart:io. Does that sound appropriate to you?

[bkonyi]

Errr, maybe. @brianquinlan will probably have some thoughts on this.

[brianquinlan]

Is there reasons why users can't use Apple Network Link Conditioner or whatever the equivalent is on their platform?

[srawlins]

That'd be a nice cheap way to close this. :D Not sure if there is an equivalent on Windows. Many Flutter developers are on Windows.